The invention relates to an R. F. balanced mixer comprising insulating substrate means having two opposed major surfaces. The mixer has first and second transmission lines, wherein the first transmission line is a balanced transmission line comprising a pair of spaced conductors on a first of said major surfaces, and wherein the second transmission line is an unbalanced transmission line comprising a strip conductor on one of said major surfaces and a ground conductor on the side of the substrate remote from said strip conductor. The mixer further includes two diodes respectively conductively connected to and between the strip conductor of the second transmission line and the pair of spaced conductors of the first transmission line. The arrangement of the first and second transmission lines and the diodes operate as a 180 degree hybrid junction. In operation the first transmission line couples a signal at a first frequency to or from the diodes, and the second transmission line couples a signal at a second frequency to the diodes and couples a signal at a third frequency equal to the difference between the first and second frequencies to or from the diodes.
Such a mixer is known from U.S. Pat. No. 3,939,430. This patent discloses a single-balanced mixer the R. F. circuit of which includes three transmission lines utilising a conducting film on a first major surface of a dielectric substrate, namely a slot line, a coplanar line (sometimes known as a coplanar waveguide), and a microstrip line. The conducting film forms a ground plane in which the slot line is defined, and forms both the central conductor and the outer, ground conductors of the coplanar line, the elongate central conductor being separated by a circumferential gap from the transversely-opposed ground conductors. The conducting film also forms the ground plane of the microstrip line which further comprises a strip conductor on the other major surface of the substrate. One end of the slot line is coupled to one end of the coplanar line, the slot and gap of the respective lines together forming a continuous conductor-free region. The other end of the coplanar line is coupled to one end of the microstrip line, the central conductor of the coplanar line being connected by a conductive pin extending through the substrate to the strip conductor of the microstrip line. At the junction of the slot line and the coplanar line, two diodes are connected in opposite electrical polarities between the central conductor of the coplanar line and, respectively, two regions of the slot line ground plane lying respectively on opposite sides of the slot. This junction arrangement operates as a 180.degree. hybrid junction (see also, for example, U.S. Pat. Nos. 3,678,395 and 4,032,849, and the paper "A 26.5-to-40 GHz Planar Balanced Mixer" by U. H. Gysel, Proceedings of the 5th European Microwave Conference, September 1975, pages 491-495).
In operation, a received R.F. signal is supplied to the junction by the slot line, while the microstrip line and the coplanar line serve to supply a local oscillator (L.O.) signal to and extract an intermediate frequency (I.F.) signal from the junction. A constructed embodiment described in the patent operated with an R.F signal frequency of 9.4 GHz and L.O. signal frequency of 7.8 GHz, giving an I.F. signal frequency of 1.6 GHz.